Multi-site pacing therapy, such as, but not limited to, multi-site left ventricular (MSLV) pacing, is becoming more common for use with patients that do not respond to traditional bi-ventricular (BiV) pacing therapy. In multi-site left ventricular (MSLV) pacing, an exemplary pacing sequence may include delivery of four pacing pulses to four different cardiac regions during each cardiac cycle. This may start with delivery of a first pacing pulse to a region within the right atrium (RA), followed by a second pacing pulse to a first region within the left ventricle (LV1), followed by third pacing pulse to a second region within the left ventricle (LV2), followed by a forth pacing pulse delivered to a region within the right ventricle (RV). Accordingly, this pacing sequence can be referred to as an RA-LV1-LV2-RV pacing sequence. Such a pacing sequence can also include one or more sense periods, such as an atrial sense period, but not limited thereto. The acronym RA, as used herein, can be used to refer to the right atrium, or to a right atrial pacing pulse, depending on context. The acronyms LV1 and LV2 can be used to refer to first and second regions within the left ventricle, or to first and second left ventricular pacing pulses, depending on context. Similarly, the acronym RV can be used to refer to the right ventricle, or to a right ventricular pacing pulse, depending on context.
The delay between the delivery of the RA pulse and the LV1 pulse, which can be referred to as the atrio-ventricular delay (AV delay) or the RA-LV1 delay, can be within a range of about 150 ms to 200 ms, but is not limited thereto. The delay between the delivery of the LV1 pulse and the LV2 pulse (which is significantly shorter than the RA-LV1 delay) can be within a range of about 5 ms to 20 ms, but is not limited thereto. Similarly, the delay between delivery of the LV2 pulse and the RV pulse is relatively short, e.g., within the range of about 5 ms to 20 ms, but is not limited thereto. The time period between delivery of the RV pulse and delivery of an RA pulse during the following cardiac cycle, which time period is a function of a pacing rate, can be within the range of about 300-1000 ms, but is not limited thereto.
Conventionally, in order to perform fast discharging of nodes within a cardiac stimulation device to achieve desired charge neutrality, the same pair of nodes that were used to deliver a pacing pulse are connected to a fast discharge pathway. Such fast discharging occurs during a fast discharge phase, which can also be referred to as a fast discharge period. Where there are relatively long time periods between pacing pulses, this conventional technique for performing fast discharging has proved successful for achieving charge neutrality. However, where there are relatively short time periods between pacing pulses, such as occurs in MSLV pacing, this conventional technique for performing fast discharging may not successful achieve the desired charge neutrality in certain situations, such as where a pacing pulse has a large amplitude and/or a large pulse width, which may necessitate restrictions on stimulation pulse programming. For example, assume that an RA-LV1-LV2-RV pacing sequence is being used. Since the RA-LV1 delay is relatively long, the conventional technique for performing fast discharging of nodes within the cardiac stimulation device can be used to fully or at least substantially achieve charge neutrality between delivery of the RA pacing pulse and the subsequent LV1 pacing pulse. However, use of the conventional technique for performing fast discharging between deliver of the LV1 and LV2 pulses may not fully discharge the nodes, due to the relatively short time period of the LV1 to LV2 interval. Accordingly, in the meantime, charges have had a chance to redistribute throughout the left ventricle and/or other cardiac chambers. Thus, charge neutrality has not proven successful in certain situations.